I think it's mostly because the 50/1.8 is recessed on the front-- it doesn't need a hood because it more or less has one built in. This is the same reason that many will tell you not to put a UV filter on it-- because it would be difficult to get anything in to break the lens. That and it is a little sillly to put nearly $60 worth of protection (hood+filter) onto a lens that cost $70.
A UV filter I can see - a quality one would cost about as much as the lens. The hood however, pricey as it may be - is a darn good idea regardless. It's not about protecting the front element as much as it is eliminating as much stray light as possible. If the Canon one is too expensive for you, bo buy a rubber one - they're great on the 35mm and 50mm.

